In an aircraft gas turbine engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against the airfoil section of the turbine blades, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward. The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. Thus, there is incentive to raise the combustion gas temperature.
The compressors and turbine of the turbine engine can comprise turbine disks (sometimes termed “turbine rotors”) or turbine shafts, as well as a number of blades mounted to the turbine disks/shafts and extending radially outwardly therefrom into the gas flow path, and rotating. Also included in the turbine engine are rotating, as well as static, seal elements that channel the airflow used for cooling certain components such as turbine blades and vanes. The airflow channeled by these rotating, as well as static, seal elements carry corrodant deposits to the non-gas path sides of turbine blades. As the maximum operating temperature of the turbine engine increases, the turbine blades are subjected to higher temperatures. As a result, oxidation and corrosion of the turbine blades have become of greater concern.
Metal salts such as alkaline sulfate, sulfites, chlorides, carbonates, oxides, and other corrodant salt deposits resulting from ingested dirt, fly ash, volcanic ash, concrete dust, sand, sea salt, etc. are a major source of the corrosion, but other elements in the bleed gas environment can also accelerate the corrosion. Alkaline sulfate corrosion in the temperature range and atmospheric region of interest results in pitting of the turbine blade substrate at temperatures typically starting around 1200° F. (649° C.). This pitting corrosion has been shown to occur on turbine blades, primarily the region beneath platforms of turbine blades. The oxidation and corrosion damage can lead to failure or premature removal and replacement of the turbine blades unless the damage is reduced or repaired.
Turbine blades for use at the highest operating temperatures are typically made of nickel-base superalloys selected for good elevated temperature strength and fatigue resistance. In addition, the turbine blade alloys are coated with environmental coatings to primarily protect the turbine airfoil and platform structures for oxidation and corrosion. These coatings may additionally be deposited on the under platform region of the turbine blade. Typical environmental coatings in wide use include MCrAlX overlay coatings (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), and diffusion coatings that contain aluminum intermetallics, predominantly beta-phase nickel aluminide (βNiAl) and platinum aluminides (PtAl). These superalloys and the existing environmental coatings used have resistance to oxidation and corrosion damage, but that resistance is not sufficient to protect them at sustained operating conditions now common in gas turbine engines. Newer superalloys have lower chromium content and are more susceptible to hot corrosion at such operating conditions.
Cooler areas on turbine blades are susceptible to hot corrosion attach. This hot corrosion attack is often particularly severe in the under platform areas where contaminants accumulate and service temperature is in the range of fastest attack. Coatings are often used to provide protection. However, these coatings can result in significant production difficulties. The most common coating for the under platform area and/or shank portion of the dovetail section is platinum aluminide. Platinum plating control in the complex geometry of the under platform area and the shank portion of the dovetail is very difficult. And PtAl coating is considered expensive. Parts with complex coating requirements require difficult masking and in-process strip cycles in order to obtain the proper coating in certain areas and avoidance in other areas. In severe applications it is observed that the PtAl has corrosion resistance that is insufficient for desired part life.
What is needed are methods of coating and coating compositions for turbine blades that: (1) provide corrosion resistance, especially at elevated temperatures where corrosion damage is more severe; (2) can be applied at a relatively low temperature with no need for in-process stripping of preselected areas; (3) can be formed by relatively uncomplicated and inexpensive methods; (4) are compatible with other part coating(s); and (5) can be used to refurbished exiting parts for continued engine operation. The present invention provides these and other related advantages.